Magnetar

A magnetar is a neutron star with an extremely strong magnetic field, the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma-rays. The theory regarding these objects was formulated by Robert Duncan and Christopher Thompson in 1992. In the course of the decade that followed, the magnetar hypothesis has become widely accepted as a likely physical explanation for observable objects known as soft gamma repeaters and anomalous X-ray pulsars.

Contents

1 The Life of a Magnetar 2 Effects of superstrong magnetic fields 3 Notes 4 External links

The Life of a Magnetar

When in a supernova a star collapses to a neutron star, its magnetic field increases dramatically in strength. Duncan and Thompson calculated that the magnetic field of a neutron star, normally an already enormous 10⁸ teslas could under certain circumstances grow even larger, to more than 10¹¹ teslas. Such a highly magnetic neutron star is called a magnetar.

In the outer layers of a magnetar, which consist of a plasma of heavy elements (mostly iron), tensions can arise that leads to 'starquakes'. These seismic vibrations are extremely energetic, and result in a burst of X-ray and gamma ray radiation. To astronomers, such an object is known as a soft gamma repeater.

It is estimated that about 1 in 10 supernova explosions results in a magnetar rather than a more standard neutron star or pulsar. This happens when the star already has a fast rotation and strong magnetic field before the supernova. It is thought that a magnetar's magnetic field is created as a result of a convection-driven dynamo of hot nuclear matter in the neutron star's interior that operates in the first ten seconds or so of a neutron star's life. If the neutron star is initially rotating as fast as the period of convection, about ten milliseconds, then the convection currents are able to operate globally and transfer a significant amount of their kinetic energy into magnetic field strength. In slower-rotating neutron stars, the convection currents only form in local regions.

The life of a magnetar as a soft gamma repeater is short: The energy release of explosions (starquakes) slows the rotation (causing magnetars to rotate much more slowly than other neutron stars of a similar age) and weakens the magnetic field, and after only about 10,000 years the starquakes cease. After this, the star still radiates X-rays, forming an object known to astronomers as an anomalous X-ray pulsar. After another 10,000 years, it becomes completely quiet. These are blockbuster detonations and some have been directly recorded, such as that at SGR 1806-20 on December 27, 2004, and more are expected to be recorded as telescopes increase in fidelity.

As of December 2004, 4 soft gamma repeaters and 5 anomalous X-ray pulsars are known, with a further four candidates in need of confirmation.

Effects of superstrong magnetic fields

A magnetic field above 10 gigateslas is strong enough to wipe a credit card from half the distance of the Moon from the Earth¹. A small neodymium based rare earth magnet has a field of about a tesla, Earth has a geomagnetic field of 30-60 microteslas, and most media used for data storage can be erased with a millitesla field.

The magnetic field of a magnetar would be lethal at a distance of up to 1000 km, by warping the atoms in living flesh².

Notes

Note 1: Origin of magnetars (http://www.cnn.com/2005/TECH/space/02/01/universe.magnets/index.html) - CNN February 2, 2005 Note 2: The Brightest Blast (http://skyandtelescope.com/news/article_1464_1.asp) - Sky and Telescope February 18, 2005.

External links

• Creation of magnetars solved (http://www.sciencedaily.com/releases/2005/02/050201193246.htm) Formed when the biggest stars explode
• NASA: "Magnetar" discovery solves 19-year-old mystery (http://science.msfc.nasa.gov/newhome/headlines/ast20may98_1.htm) Citat: "...suggested a magnetic field strength of about 800 trillion Gauss...").
• Robert C. Duncan, University of Texas at Austin: 'Magnetars', Soft Gamma Repeaters & Very Strong Magnetic Fields (http://solomon.as.utexas.edu/~duncan/magnetar.html)
• NASA Astrophysics Data System (ADS): Duncan & Thompson, Ap.J. 392, L9) 1992 (http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1992ApJ...392L...9D)
• NASA ADS, 1999: Discovery of a Magnetar Associated with the Soft Gamma Repeater SGR 1900+14 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999ApJ...510L.115K&db_key=AST&high=3eb1acc4ea06362)
• Chryssa Kouveliotou, Robert Duncan, and Christopher Thompson, "Magnetars," Scientific American, Feb. 2003, pp. 34-41 (http://www.cita.utoronto.ca/~thompson/magnetar.pdf) (PDF)
• Robert C. Duncan and Christopher Thompson, "Formation of Very Strongly Magnetized Neutron Stars: Implications for Gamma-Ray Bursts," Astronomical Journal, vol. 392, no. 1 (June 10, 1992), pp. L9?L13. (http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1992ApJ...392L...9D)bg:Магнетар

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